Stainless steel generally owes its good corrosion resistance to a passivating film of chromic oxide. It has been found that sintered stainless steel has a lower corrosion resistance than the wrought metal even when passivated, for example by treatment with nitric acid. There may be several reasons for this, including inadequate passivation and an increase in pitting corrosion caused by the porosity of the sintered material. These problems are particularly serious in the case of attack by acid solutions in non-oxidizing conditions or e.g. hot chloride solutions. In normal circumstances, while acid attacks the passivating film of oxide, the film is constantly being replenished. If there is de-aeration of the solution, however, breakdown of the film occurs rapidly.
Sintered stainless steel has therefore been thought unsuitable for use in the presence of such corrosive materials. Where a sintered material is necessary, e.g. for use as a filter, substances such as sintered glass may be used but these present their own problems. In the food industry in particular it may be necessary to ensure that particles or fragments of a sintered filter do not pass into food products--the use of sintered stainless steel filters might therefore be much preferable to the use of sintered glass filters as the detection of stainless steel fragments would be more easily achieved than that of glass fragments.
It has been proposed, in U.K. Patent No. 2004454B to mold a sausage in a sintered mold and to pass acetic or another acid through the wall of the mold to treat the surface of the sausage. In tests it was found that with molds of sintered stainless steel, after using acetic acid, the molds became clogged with a dark deposit and virtually unusable.
It has also been found that when using sintered stainless steel filters in the filtration of whisky, the whisky becomes discolor. Indeed, in a test, when a piece of sintered stainless steel was immersed in whisky, discoloration was rapid. Investigations have shown whisky to have a relatively low pH of say 3.6, with between 80% and 90% of the acid content being acetic. It is thus considered that the problem encountered may correspond to that in the case of sausage molding as described above.
There is thus a distinct problem in that while sintered stainless steel might be considered a useful material in for example the processing of food or drink, it suffers badly from corrosion problems. These have been particularly noted in the case of acetic acid and would be expected with other acids.
The object of the invention is therefore to reduce or eliminate these substantial corrosion problems.